This does not know anything about arti, only about TOML and Config.

Code motion, plus necessary import adjustments.

This does involve unwrap, but of course that can't fail unless the
formats fail, which would already panic (that's implied by format!).

The type of ret.map_err(codec_err_to_chan)? is ().  ISTM that
writing `let () = ` makes it clear that there is nothing there,
but the lint forbids this.

This lint is warn by default and trips here for me on current nightly.

It seems wrong to me.  We should be able to make it clear to the
reader that there is nothing here - note how this differs from the
lines below where Ready contains msg.  A let () binding is a good way
to do that.

I think the lint allow ought to be added everywhere, but that doesn't
seem easy right now - see this issue about maint/add_warning:
   tpo/core/arti#469

Improvements prompted by clippy, and disable one lint

See merge request !497

arti-config: Move cmdline to tor-config

See merge request !498